This past July was the hottest single month in Earth’s recorded history, but warming isn’t the only danger climate change holds in store.

Recent years have seen a dramatic increase in the simultaneous occurrence of extremely cold winter days in the Eastern United States and extremely warm winter days in the West.

“There’s this idea that the past few winters were more extreme than usual, particularly since the conditions in the East and West were so different,” says Noah Diffenbaugh, associate professor of Earth system science at Stanford University and senior author of the new study in the Journal of Geophysical Research Atmospheres.

“Looking back at temperature data from the past 35 years, we’ve found that in fact 2013-2014 and 2014-2015 did have the biggest difference in winter temperature between the East and West.”

Be prepared

Understanding the physical factors driving extreme weather could provide policymakers with more reliable information with which to prepare for short-term weather disasters such as blizzards and cold snaps.

Long-term planning and development would benefit, too. For instance, understanding the likelihood of droughts could help engineers better plan the development and management of infrastructure to provide reliable water supplies.

In the past three years alone, the combination of heat-related drought in the West and Arctic conditions in the East have pinched the national economy, costing several billion dollars in insured losses, government aid, and lost productivity.

When such weather extremes occur at the same time, they threaten to stretch emergency responders’ disaster assistance abilities, strain resources such as interregional transportation, and burden taxpayer-funded disaster relief.

That cold is climate change

The occurrence and severity of “warm West, cold East” winter events increased significantly between 1980 and 2015, partly because the winter temperature has warmed more in the West than in the East, increasing the odds that warm days in the West coincide with cold days in the East.

Along with warming of the West, a “ridge-trough” pattern of high atmospheric pressure in the West and low atmospheric pressure in the East has also been producing greater numbers of winter days on which large areas of the West and East experience extreme temperatures at the same time.

“What we’ve found is that this particular atmospheric configuration connects the cold extremes in the East to the occurrence of warm extremes in the West,” says lead author Deepti Singh, a former graduate student in Diffenbaugh’s research group who is now at Columbia University.

Despite long-term warming across most of the globe, some regions can experience colder than normal temperatures associated with anomalous circulation patterns that drive cold air from the poles to the mid-latitudes. In fact, circulation patterns that facilitate such extremes are potentially a response to enhanced warming.

“Although the occurrence of cold extremes is often used as evidence to dismiss the existence of human-caused global warming, our work shows that the warm West, cool East trend is actually consistent with the influence of human activities that have modified Earth’s climate in recent decades,” Singh says.

That said, the simultaneous occurrence of extreme western warmth and extreme eastern cold will likely decrease if global warming continues through the 21st century, because warming of winters in both the West and East will likely reduce the occurrence of cold winters in the East. Still, the researchers project that some extremely cold events will still occur even with high levels of global warming.

“We can absolutely expect further increases in hot events if global warming continues,” Diffenbaugh says. “But our results also highlight how complex climate change can be. We should be prepared for both warm and cold extremes—sometimes simultaneously—now and in the future.”

Other researchers from Stanford, Columbia, Northwestern University, and the University of California, Los Angeles, are coauthors of the study.